The tank is one-way-or-the-other connected to:
- the compressor (duh), through a one-way check valve,
- the outlet (duh again),
- the pressure switch,
- a pressure relief valve (the safety valve),
- a pressure gauge.

My compressor has a "christmas tree" fitting that all of these connect
to. I'm thinking that your 4-way is similar. Maybe:

gauge
|
compressor - (switch) - outlet
|
tank

Bob